The tremendous growth of the semiconductor industry has required suppliers of components used in the fabrication of silicon integrated circuits (ICs)s to come up with improvements in their products to increase the productivity of ICs without increasing their cost. One example of the need for such an improvement is to increase the wear life of the capillaries or weld tip guides used in the wire ball bonding operation of the IC fabrication process. In such an operation, a clean, bare fine electrical conductor such as gold, aluminum or silver is attached between a bonding pad on the IC chip and pins on the chip support. Specifically, the fine electrical conductor wire or filament, usually about 0.7 to 5 mils in thickness (0.0007 in. to 0.005 in.), is fed through the bonding tool known as the capillary. In the case of the currently most popular bonding means, i.e. thermosonic bonding, the capillary is held in an ultrasonic transducer horn, a ball is formed at the tip end of the capillary by melting the wire using an electrical spark, and the ball is bonded between the bonding pad and pins by a combination of thermocompression and ultrasonic techniques; see Microelectron Manufacturing and Testing, August 1984, pages 53-54. Thermosonic bonding is done in bonding machines which include relatively simple manually operated types to more advanced high speed fully automatic wire bonders, which include microprocessors for pattern recognition and maintaining bonding schedules; see the product brochure for the Model 2460 thermosonic gold ball bonding machine by Hughes Aircraft Company.
The majority of capillaries produced in the semiconductor industry today and used in such thermosonic bonders are made of molded alumina. Such alumina capillaries are low in cost and can be discarded after use rather than having to clean or to reclaim them. However, the chief drawback of molded alumina capillaries is their relatively short wear life in high speed electronic flame off (EFO) bonding applications due to their lower resistance to thermal shock. This results in frequent replacement which lowers overall productivity during the bonding operation. Diamond, ruby and sapphire tipped capillaries have the requisite long wear life, but they do not sell at competitive prices, must be cleaned during use and should be reclaimed after their useful life.
Alumina capillaries are formed by a process which comprises mixing an aqueous slurry of alumina with a proper binder, molding the mixture into a compact shape over a mold pin and sintering the alumina to form the finished product. It is the binders used in the forming process that contribute to failures of the capillaries during use.
A process for producing capillaries by chemical vapor deposition (CVD) techniques is well known in the art; see in Holzl et al, U.S. Pat. No. 3,472,443. In the process, a molecularly bonded chemical vapor deposited metal, which is non-reactable with the hot ball and wire, is coated onto a mold and the mold is readily removed from the resulting capillary. Although the patentees disclose that any suitable highly refractory hard material having a Vickers microhardness in excess of about 400 kg/mm.sup.2 can be used to produce the capillaries, they disclose that particularly suitable materials include tungsten, rhenium and molybdenum, and tantalum and alloys thereof, such as tungsten-carbide, tungsten-molybdenum and tungsten-rhenium, as well as tantalum-carbide and columbium-carbide. Such capillaries are unsuitable in thermocompression and ultrasonic bonders which use electric generators for producing the hot ball and thus, require resistivities of no less than 0.1 ohm-cm. No suggestion is made in this patent of preparing silicon carbide capillaries by CVD techniques and hence, the patentees had no recognition of the problems which are associated with preparing such capillaries and which had to be overcome to produce CVD silicon carbide capillaries.
It also known to manufacture capillaries from relatively low melting point cementitious metals, such as cobalt, titanium and nickel. Such capillaries must be sintered using binder materials which have the disadvantage of reacting with the fine wire of the bonding operation causing loss of bond strength. Electrochemical deposition methods for forming capillaries have been used to produce nickel, copper and iron capillaries which are all unsuitable because these metals react with the fine wire.
An entirely different bonding method is disclosed and claimed in U.S. Pat. No. 4,171,477 which requires that the capillary be electrically conductive. The patentee discloses that in the case of bonding gold wire, capillaries are used comprising titanium carbide. Although for other fine wire materials, they suggest the use of graphite, silicon carbide, zirconium boride-silicon carbide, and silicon crystals. This patent does not include any details of the method for making such a capillary.
Even though the prior art may have suggested CVD silicon carbide capillaries to those skilled in the CVD art, until the present invention there was no disclosure in the prior art of silicon carbide capillaries having the necessary physical properties for use in EFO type of bonders nor any disclosure of a method for manufacturing silicon carbide capillaries having such properties using CVD techniques.